Abstract

We propose a technique for studying simultaneously
observed ULF activity in the Pc5 frequency band, global patterns
of field-aligned currents (FAC) derived from the IZMEM model
driven by IMF, and maps of dayside magnetospheric regions obtained
from DMSP charged particle flux measurements. This technique
produces a sequence of two-dimensional "snap-shots" of the
FAC distributions with overlapped DMSP satellites tracks along
which we obtain a projection of magnetospheric regions onto the
ionosphere. The 2-D distribution of the ULF spectral power is
inferred from the worldwide array of geomagnetic stations. For
most of the analyzed morning sector events the region of downward
FAC maps together with the
low latitude boundary layer (LLBL)
projection to the polar
ionosphere, whereas the upward FAC region coincides with the
central plasma sheet (CPS)
projection.
Simultaneous occurrence of two source regions of the ULF activity is observed:
one is located in the early morning/afternoon (magnetic local time) hours at
latitudes 65o-70o, and the other is observed
near noon at latitudes
75o-79o. The morning sector ULF intensity peaks
equatorward of the R1 FAC. The near-noon Pc5 pulsation power peak
coincides with the equatorward boundary of the LLBL, whereas a
resonant maximum of Pc5 pulsations in the early morning hours corresponds
to the CPS region.

1. Introduction

The interaction of solar wind plasma flow with the
Earth's magnetosphere can be
considered as a giant, natural magnetohydrodynamic (MHD) generator, which
produces the large-scale, quasi-steady 3D current system at high latitudes.
This system includes the field-aligned currents (FAC), or Birkeland
currents, in the magnetosphere, coupled with the closure (horizontal)
currents in
the ionosphere. The intensity and spatial distributions of these
current systems
are controlled to a considerable extent by the interplanetary magnetic field
(IMF). Much of the transfer of energy and momentum from the solar
wind through the
magnetosheath into the magnetosphere occurs at the dayside magnetospheric
boundary layers. Mapping of magnetospheric boundary layers to low altitudes
bears substantial uncertainty because of the uncertain topology of the entire
magnetosphere, although it is clear that these spatially vast regions map
into a
very limited area around the low-altitude cusps. This mapping was studied by
using advanced magnetic field models, low-altitude measurements of
charged-particle precipitation, visible auroral emissions, radar observations,
and other sources of information.

The global magnetosphere-ionosphere current system, as it is reconstructed from
magnetic field measurements either on the ground or on low-altitude satellites,
can be decomposed into several subsystems. In the dawn/dusk sectors, the
ionospheric DP1/2 (or field-aligned R1/R2) current system dominates.
This current system consists of two longitudinally elongated current sheets
with downward (upward) FAC in the poleward sheet (R1) and more wide upward
(downward) FAC in the equatorward sheet (R2) at dawn (dusk). This system is
intensified under conditions of southward IMF ( Bz < 0 ).

In the dayside cusp/cleft region, the ionospheric disturbance
polar driven by
y component of IMF (DPY) current system is
observed if the IMF has a substantial azimuthal component
[Troshichev et al., 1997].
In its simplest form, the DPY
current
system is an east-west oriented Hall current
which is formed between the ionospheric ends
of two FAC sheets. The equatorward
sheet may be an extension of the R1 current,
while the poleward sheet is located
at higher latitudes, in the cusp/mantle region.
It is often referred to as R0 FAC.
Position and intensity of this current system are controlled by the IMF,
predominantly by the azimuthal component,
By, and to a lesser degree by the
vertical component,
Bz.
Poleward of the cusp, the
current system driven by northern
Bz (NBZ)
current system resides, most evident during
periods of northward IMF,
Bz>0. The NBZ current system is located near noon
at geomagnetic latitudes higher than 80o, with the upward prenoon FACs
and downward postnoon FACs.

The energy transfer from the solar wind plasma into the magnetosphere and
ionosphere has a turbulent character.
Thus, it might be expected that in the key
boundary regions, electromagnetic and
magnetohydrodynamic noise can be generated.
The occurrence of natural magnetospheric MHD
waveguides and resonators may result
in the noise's partial filtering producing quasiperiodic pulsations. Indeed,
at high latitudes, intense quasiperiodic ULF (ultra-low-frequency) pulsations
of
the geomagnetic field in the Pc5 range (1-10 mHz)
are commonly observed, but the
exact physical mechanism for these ULF disturbances has not yet been established.
The common view is that the main source of dayside Pc5 waves is the
Kelvin-Helmholtz (K-H) instability at the flanks of the magnetosphere, excited by
the solar wind flow. The velocity shear may exist at interfaces between other
magnetospheric boundary regions, thus being the probable source of the
K-H-generated disturbances.

Inside the magnetosphere, these disturbances are transformed into more regular,
quasi-monochromatic Pc5 pulsations under the influence of magnetospheric
resonance effects. The position of the resonance is determined by the match
between the local Alfvén frequency and
the frequency of an external source,
irrespective of a particular source mechanism. According to this notion, the
latitude of maximal ULF intensity is determined by the features of the
magnetospheric plasma distribution.

An intriguing but still not resolved problem is the identification on the
ground of specific ULF wave signatures of boundary phenomena.
Early studies
[Olson, 1986;
Rostoker et al., 1972;
Troitskaya and Bol'shakova, 1977, 1988]
suggested that a probable source of the dayside
high-latitude long-period
pulsations was related to the cusp,
the region of direct penetration of
turbulent magnetosheath plasma into the magnetosphere/ionosphere. The broadband
disturbances in the period range of 3-15 min (named
irregular pulsations at
cusp latitudes (IPCL)
[Troitskaya, 1985]
or broadband Pc5 pulsations
[Clauer and Ridley, 1995];
Engebretson et al., 1995])
were also claimed to be
typical features of the dayside cusp/cleft. Occurrence of ground-based pulsations
has been suggested to be used for monitoring
the dynamics of the cusp/cleft
region
[Bolshakova et al., 1988;
Kleimenova et al., 1985;
McHenry et al., 1990].
Later,
McHarg et al. [1995]
and
Lanzerotti et al. [1999]
found that small-amplitude
quasi-monochromatic Pc5 waves at the dayside might be a signature of near-cusp
closed field lines and could be used as cusp discriminators. However, this goal
can hardly be achieved by simple means. Observations at the MACCS
(cusp-centered) array showed that the dayside broadband ULF activity is
dominated by temporal variations across a large longitudinal extent
[Engebretson et al., 1995].
Szuberla et al. [[2000]
succeeded to identify a
cusp signature in coherent Pc5 waves using polarization spectra.

Various hypotheses have been suggested for interpretation of the
high-latitude ULF disturbances, including a fluctuating component
of FACs or precipitating electrons [Engebretson et al.,
1991];
fluctuations of the cusp-related current
system
[Olson, 1986],
and the K-H instability in the region of the convection
reversal boundary
[Clauer et al., 1997]
or in the inner part of the LLBL
[Lee et al., 1981].

The attempts to relate the location of ULF waves observed on the ground to
magnetospheric boundary regions have been done so far either on a statistical
basis, with all the inherent uncertainties of using average locations
of boundaries, or on a very limited regional basis defined by a particular
magnetometer array. To put ULF studies in a more global magnetosphere/ionosphere
context, it is necessary to develop an approach that would enable one to study
simultaneously the ULF global pattern together with some proxies of ionospheric
electrodynamics and identification of ionospheric projections of dayside
magnetospheric regions. A possible tool for these studies is suggested in this
paper. We have combined observations from about 50 magnetometers in the northern
high latitudes to produce an instantaneous (on the scale of the spectral window)
global map of ULF wave power and have related this to magnetospheric regions
defined either using DMSP particle data or a statistical FAC pattern that
depends on the IMF measured at the time of the observation and propagated to
the bow shock location.
For the first time,
this allows
a view of the global distribution of ULF power
and its relationship to magnetospheric regions. This tool has been applied to
the problem of how the Pc5 ULF pulsations observed in the dayside polar region
are related to the magnetospheric regions and global current systems
(R1/R2, R0, and NBZ).

2. Empirical-Analytical Model of High-Latitude Global
Electrodynamics

Long-term observations at high-latitude magnetic observatories
established a reliable connection between the IMF and ionospheric current
systems, which resulted in several empirical-analytical models. One of the
approaches, the IZMIRAN Electrodynamics Model (IZMEM), developed at the
Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation
(IZMIRAN)
[Feldstein and Levitin, 1986;
Papitashvili et al., 1994],
utilizes a
linear regression relationship between the IMF and ground-based geomagnetic
disturbances at high latitudes. The IZMEM model has recently been recalibrated
utilizing the DMSP electrostatic potentials
[Papitashvili et al., 1999];
the
Web-based interface to the IZMEM model has been made available at
http://www.sprl.umich.edu/MIST/limie.html. With the use of this model, the
ionospheric electrodynamic pattern for a given IMF intensity and orientation
can be calculated and mapped over both the northern and southern polar
regions using a statistical model of ionospheric conductivity
[e.g.,
Robinson and Vondrak, 1984;
Wallis and Budzinski, 1981].
The algorithm incorporated in the
model separates the ground magnetic variations into internal and external parts,
restores electric potential and horizontal currents in the ionosphere, and
finally determines the FACs in and out of the ionosphere. The IZMEM model
does not require collection of in situ ground-based geomagnetic data for the
event under investigation or selection of a magnetically quiet period to calculate
geomagnetic disturbances. This distinguishes the IZMEM from other algorithms
such as the "magnetogram inversion technique"
[Mishin et al., 1980],
the KRM method
[Kamide et al., 1981],
and the AMIE technique
[Richmond and Kamide, 1988].

The IZMEM model allows the user to obtain instantaneous distributions of
ionospheric electric potentials or FACs for a given IMF during the time interval
under investigation. However, IZMEM is an empirical model, so physical
mechanisms of predicted 3D systems and identification of the basic current
elements within the magnetospheric regions have not been considered in
this model.

3. Ground-Based Magnetometer Arrays

Figure 1

The existing global network of magnetic stations at high latitudes,
shown in
Figure 1,
forms several latitudinal profiles:
(1) CANOPUS (http://www.dan.sp-agency.ca/www/) is a network of 13 automatic
stations deployed over westcentral Canada with a sampling period of 5 s.
(2) MACCS (http://space.augsburg.edu/space/)
is a network with 12 identical
fluxgate magnetometers with 0.5-1.0 s
sampling deployed in the Canadian Arctic,
which includes one station in the polar cap and longitudinal profiles along
geomagnetic latitudes ~79o N and ~75o N.
Together, MACCS and CANOPUS form three meridian profiles along corrected
geomagnetic longitudes, ~15o (MC-East), ~335o (MC-Center),
and ~315o (MC-West).
(3) The Greenland coastal chain
(http://www.dmi.dk/projects/chain/)is an array
of 17 magnetic stations with 20-s sampling rate, deployed along the west
(~40o) and east (~95o) coasts of Greenland. It is
augmented by MAGIC
(http://www.sprl.umich.edu/MIST/), a magnetometer
array on the Greenland ice cap (at ~
60o magnetic longitude).
(4) IMAGE (http://www.geo.fmi.fi/image), an auroral and sub-auroral network
of 24 magnetometer stations with 10-s sampling rate,
clustered along the Scandinavian meridian (~105o CGM longitude).

With these arrays, the spatial coverage of stations is dense enough to
enable us to proceed from the analysis of 1D latitudinal profiles to the
examination of 2D patterns. The pertinent technique is described further below.

Charged-particle precipitation characteristics seem to be the best low-altitude
means to categorize the ionospheric projection of the magnetospheric boundary
layers
[Newell and Meng, 1988].
In this study observations from the
Defense Meteorological Satellite Program (DMSP) satellites F10-F12 are used.
The automated dayside region identification program distinguishes
magnetospheric regions through the characteristics of precipitating electrons
and ions (in the 30 eV to 30 keV range) at DMSP altitude (~800 km)
[Newell et al., 1991a, 1991b].
The classification scheme is described in
Appendix A. The region identification
database is given as a set of files with
the Universal Time (UT) of the crossing of each boundary. The crossings are
further tagged with the geomagnetic latitude and magnetic local time and with
the geocentric coordinates.

5. Visualization and Mapping Technique

We develop a technique for simultaneous mapping (as a sequence of 2-D
"snap-shots") of the ionospheric electrodynamic pattern, predicted by the IZMEM
model, and of the ULF spectral power. We consider it as a first step
toward performing a dynamical 2D analysis of ULF pulsations.

The program decimates the original geomagnetic time series to a common sampling
period of 20 s. At each station, the
H component
is transformed into the
frequency domain via an FFT over a gliding window, and the spectral power within
a selected frequency band is estimated. These data are used to construct the 2D
spatial distribution of ULF power
( H component) for a particular time interval.
For the gridding of irregularly spaced data the IDL routine CONTOUR has been
used which handles this problem by constructing a Delaunay triangulation.

In subsequent analysis, a 1-hour window and the 2.0-8.0 mHz
frequency band are
used. The time interval on the plots is indicated by its onset, that is,
"1000 UT"
denotes "1000-1100 UT". The ULF power within each time interval is normalized
with
the maximum of the whole day. It is indicated on each plot and may be used
as an indicator of ULF intensity in each snap-shot.

For the calculation of 2D spatial distributions of FACs over the high-latitude
ionosphere as predicted by the IZMEM model, the necessary data are taken
automatically from the OMNI database of 1-hour means of the IMF/solar wind
parameters. Thus, the semiempirical model IZMEM driven by the IMF parameters
produces snap-shots of 2D polar plots with the spatial structure of FACs
throughout the dayside high-latitude ionosphere. The upward (negative) FACs
are assumed to be transported by precipitating electrons whereas the downward
(positive) FACs are carried by upward flow of ionospheric electrons.

To establish a correspondence between the spatial distribution of ULF
intensity, FACs, and magnetospheric boundaries, we overlay on the plots
available DMSP satellite tracks with the results of automated identification of
magnetospheric boundaries. The ground track of each orbit is plotted in
geomagnetic coordinates with the footpoints of the magnetospheric regions marked
by diamonds (cusp), squares (LLBL), crosses (BPS), triangles (CPS) and
pluses (mantle). Thus, this track clearly indicates the position of the
ionospheric projection of each magnetospheric region.

6. Case Studies With New Mapping Technique

This technique is used to analyze the relationship between the large-scale
current systems, the intensity of ULF waves and the relation of both
parameters to the dayside magnetospheric boundaries. The suggested mapping
technique can be an effective tool for the study of the following questions:

1. What is the correspondence between the basic FAC regions (e.g., predicted by
the IZMEM model) and dayside magnetospheric regions?

2. What is the correspondence between the position of the spatial peak in the ULF
Pc5
power distribution and FACs?

3. In which magnetospheric regions are the probable sources of ULF activity are located?

Special attention is paid to the noon and morning MLT sectors as these are the
sectors of the most intense ULF activity in the Pc5 band. Hourly patterns for the
following three days have been selected (rather arbitrarily) for analysis:
(1) 26 November 1995 (day 360). The IMF
Bz is slightly negative
( -2 nT)
during this day. Northward
Bz excursions at ~0400 UT and ~1700 UT
stimulated substorms observed at ~0400 UT (GCA, MACCS, and CANOPUS),
and at ~1800 UT (IMAGE).
(2) 18 February 1995 (day 049).
At this day the IMF
Bz was slightly northward and
By varied from
+5 nT to
-4 nT at ~0300-0400 UT,
then remained near zero; and
(3) 24 November 1995 (day 328).
The OMNI IMF data indicate distinct periods with
By<0 (down to
-5 nT) for different
Bz conditions.
Southward deviations of
Bz cause substorms at ~1545 UT (CPMN) and ~1900 UT
(IMAGE).

The IMF
By changes during the latter two days should produce variations
of the
DPY current system;
thus a possible coupling of dayside Pc5 activity and DPY
current intensification could be examined.

6.1. Comparison of FAC Regions as Derived From the IZMEM
Model
With Magnetospheric Regions

Figure 2

Figure 2 shows the distribution of FAC in the dayside ionosphere
with superposed
tracks of DMSP satellites. This event,
18 February 1995,
presents a typical pattern of
FAC spatial structure as derived from the IZMEM model: the occurrence of the NBZ
current system near noon at latitudes higher than
80o CGM latitude, with the
upward prenoon and downward postnoon FACs, and below
80o a typical R1
structure with downward current on the dawnside and upward current on the
duskside. At 1600 UT in the prenoon hours (~10 MLT), the upward R2 current
coincides with the CPS projection, whereas the most intense part of the R1
downward FAC is mainly in the LLBL, with some admixture of the BPS projections.
This correspondence is in accord with the common notion that LLBL is the source
of the region 1 current system. The NBZ system is clearly seen because of IMF
Bz>0, and DMSP-derived projections show that the upward
NBZ current is located in the mantle. Similar observations have been made in
a large number of events.

6.2. Correspondence Between ULF Activity and FAC

Many snap-shots show that at least two sources in the Pc5 band may operate
simultaneously. One of them is located in the morning sector and generates
quasi-monochromatic Pc5 pulsations as the examination of magnetograms
(not shown)
indicates. The other is located near noon. The magnetograms show that the Pc5
activity associated with the latter is characterized by longer periods and more
irregular appearance compared to morning Pc5 activity.
This second source of broadband Pc5 (IPCL) was often attributed to the
ionospheric footpoint of the cusp.

Figure 3

The correspondence between the morning sector Pc5 wave power
distribution along
the Greenland West Coast array and the IZMEM-derived FAC can be seen in
Figure 3
for the event of 26 December 1995
at 1300-1400 UT (no DMSP tracks for this interval).
The paucity of magnetic stations prevents us from resolving the 2D wave power
distribution for this event.
Figure 3
shows that morning Pc5 are excited equatorward of the R1 current. Thus,
the location of these pulsations may be related to the auroral electrojet flowing
between the R1 and R2 current sheets.

After having analyzed all days we found no clear correspondence between the
near-noon Pc5 peak and high-latitude current system features (such as DPY).

6.3. Identification of Source Regions of ULF Activity

Superposition of ULF power spatial distribution snapshots and DMSP tracks
makes it possible to identify the probable source region of ULF activity.
However, one should keep in mind that some peaks may not be directly related
to a primary source but to the region of local resonant amplification of
Pc5 waves. Thus, this region can be considered as a "secondary" source of
these waves. Commonly, the waves in the resonant region are more intense and
more monochromatic than those in the primary source.
Some examples are given below.

Figure 4

On 26 December 1995, at 1800-1900 UT (Figure 4)
two peaks of ULF wave power are
observed: in the morning hours and near noon (especially evident in
Figure 4b).
The maximum of monochromatic Pc5 power (the region of resonant
amplification) in the early morning hours at ~
72o is equatorward of the
R1 current zone, as in the previous event,
26 December 1995.
The DMSP magnetospheric
region identification algorithm indicates that the center of morning Pc5 wave
power is located inside the CPS region. At the same time, weaker near-noon and
evening spatial maxima can be seen. However, there are no DMSP passes over
these centers of the ULF activity to identify the magnetospheric region of their
source.

Figure 5

Figure 6

For another event
(26 December 1995,
1400-1500 UT),
Figure 5
shows
the ULF power density
profile along the MC-Center stations with DMSP tracks overlaid. The morning Pc5
pulsation maximum is near the CPS/BPS boundary projection at ~72o.

During the
24 November 1995,
1800-1900 UT event (Figure 6), DMSP passes through the
near-noon maximum of ULF activity. The magnetograms (not shown) indicate that
this ULF activity is broadband Pc5, commonly named cusp-related Pc5 or IPCL.
The latitude of the power peak, 78o, is higher than the typical latitude
of morning Pc5 waves. As DMSP data indicate, the source region of the near-noon,
broadband Pc5 pulsations for this event are not located in the cusp
but coincide with the equatorward boundary of the LLBL.

7. Discussion

Our study shows that the IZMEM-predicted location of R1 FACs coincides mostly
with the LLBL. This result agrees well with the
common view about the connection between R1 currents and the LLBL, namely, that
the R1 FAC flows near or at the magnetosphere/LLBL interface, as has previously been
suggested
[e.g., Hones,1983].
This correspondence gives additional support to the
physical background of the IZMEM model. At the same time, R2 FAC in the morning
sector are located mostly in the CPS region.

The relationship between large-scale FACs and ULF oscillations was studied by
Potemra et al. [1988],
who examined conjunctions between the polar orbiting Viking
and AMPTE CCE in the equatorial orbit. The Viking particle observations confirmed
that the R1/R2 interface mapped closely to the interface between the LLBL
and CPS. Field line oscillations in the Pc5 band were detected on the same field
lines that guide the R2 FACs (flowing away from the ionosphere) in the morning
sector. They extended from the interface of the R1 and R2 current system, close
to the LLBL/CPS interface, to the lowest L crossed by Viking. R1 currents were
suggested to be associated with and possibly have their source in the LLBL,
whereas the R2 currents should be associated with CPS.

In the morning sector, the peak of ULF resonant waves is commonly located
at the equatorward boundary of the downward FAC (R1).
This location may correspond to the R2 current system, in line with the observations
of
Potemra et al. [1988]
or to the position of a westward auroral electrojet
(Hall current), located between the R1 and R2 FACs.
The accuracy of our ionospheric model is not sufficient to discriminate
between these possibilities.
In the morning/dayside sector, the latitude where the Pc5 wave power reaches peak
amplitudes coincides with the latitude where the westward electrojet is most intense.
This is in line with earlier observations of
Lam and Rostoker [1978]
and, more
recently,
Pilipenko et al. [[2001].
This effect still lacks further observational confirmation and satisfactory
interpretation.

The observed correspondence between the morning sector Pc5 wave power peak and
the CPS projection may indicate where in the magnetosphere the region of resonant
wave conversion is located. It is worth mentioning that
Yahnin and Moretto [1996]
found that centers of travelling convection vortices (TCV) in the ionosphere also
mapped to the CPS. The preference of both wave and transient responses to the
same region may evidence the occurrence of favourable conditions, e.g.,
density
gradients, in this region.

Being a statistical model, the simple IZMEM technique can provide
only a hint on the possible location of basic electrodynamic features
for case studies.
However, the technique enables us to substitute IZMEM with any other
ionospheric model
[e.g., Weimer, 1996].
For further case studies, we intend to incorporate
the
possibility to use results of the more advanced AMIE model or Ultraviolet
Imagers in our mapping technique.

Early studies of dayside ULF activity at high latitudes gave support to the view
that long-period irregular variations were closely associated with the
cusp/polar cap interface and thus could be used as a simple indicator of dayside
cusp position and polar cap boundary. However, further studies of high-latitude
broadband wave activity on the dayside
[Engebretson et al., 1995]
showed that ULF
wave activity is not associated in a simple way with boundary layer or cusp.
Szuberla et al. [[2000]
used polarization analysis and identified small-amplitude
coherent Pc5 waves as cusp signature. Larger-amplitude pulsations bounding the
former were representative of the boundary layers and showed correlated time
dependence across several hours of local time. They obscure the cusp signature
in ordinary power spectra analysis. A cause for these widespread temporal
variations, as well as their source, has not yet been identified.
The power spectra used in this study do not discriminate between polarized and
nonpolarized pulsations, which may be a reason why we have not seen
a correspondence between the proper cusp and ULF wave intensity.

In contrast to the approach in this paper, the search for specific ULF signatures
of boundary phenomena was
based on data from isolated
stations with limited latitude/longitude coverage
in most previous studies. At subauroral stations a
persistent occurrence of quasi-monochromatic Pc5 pulsations are observed, mostly
in the early morning hours during a substorm recovery phase. At higher latitudes,
irregular long-period variations were observed at near-noon hours. However, some
case studies with more extended arrays showed a regular transition from irregular
broadband (IPCL) pulsations at high latitudes to more intense and monochromatic
Pc5 pulsations at lower latitudes
[Clauer et al., 1997;
Pilipenko et al., 1998].
These events indicated that Pc5 and IPCL pulsations are not separate wave
phenomena but are
manifestations of the same wave process, whereas the difference
in their appearance is related to the resonant amplification deeper
in the magnetosphere, probably on closed dipole-like field lines.
Thus, simultaneous occurrence of IPCL and Pc5 near noon may signify a situation
where both the ULF driver and the resonant response are observed on the ground.
However,
Kleimenova et al. [1998]
presented ULF events where IPCL and Pc5 were
not related to each other. Therefore, the problem of the IPCL/Pc5 coupling needs
further investigation.
The region of the possible ULF driver is difficult to identify because the secondary
maximum in a resonant region can be higher than the primary maximum in the source
region.

Among all the considered events, we never observed a spatial correspondence
between the cusp proper and the ULF activity peak. In the event shown here, we
found that the peak of ULF activity near noon indeed mapped to the inner
boundary of the LLBL. The occurrence of irregular magnetic variations with time
scales 5-10 min inside the LLBL was detected in satellite data by
Takashashi et al. [1991].
Coincidence of the probable source region of near-noon Pc5 pulsations with the
LLBL projection agrees with an event with simultaneous radar and magnetometer
observations studied by
Clauer et al. [1997],
who attributed the source of these
pulsations to K-H instability excitation at field lines which map to the reversal
boundary of ionospheric convection which is associated with the LLBL.
Though a correspondence between ULF waves and LLBL needs further statistical
investigation, it seems that the widely used term "cusp-related pulsations" is
likely not adequate; probably, the term "boundary layer associated broadband
ULF activity" would be more adequate.

Observations of morning and postnoon Pc5 led earlier workers to the conclusion
that the K-H instability at the magnetopause or LLBL is a likely candidate for
Pc5 drivers. Later, indications were found that impulsive variations of the
dynamic pressure of the solar wind and FTE constitute another possible source of
Pc5 wave packets in the magnetosphere. Thus, the intensity of Pc5 pulsations can
be considered as an indicator of the level of turbulence in the boundary layers.
Our analysis often revealed the simultaneous occurrence of three
regions of ULF
intensification: morning sector, near-noon, and postnoon hours. They may be
hardly ascribed to the same driving mechanism, such as the K-H instability.
Moreover, the assumption of the K-H instability as a universal driving source of
geomagnetic pulsations meets some difficulties because the linear theory
predicts the predominant growth of wave disturbances with small lateral scales.

The same K-H instability can be hardly applied to the near-noon broadband Pc5
because in this region the velocity of the magnetosheath plasma flow is low.
The Pc5/IPCL activity at near-noon hours could be impulsively driven pulsations
which occur in response to the magnetosheath plasma discontinuities and
buffeting. In line with this idea, analysis of a series of IPCL bursts by
Kurazkovskaya and Klain [[2000]
showed that these signals possess rather
distinctive features, typical for a system near a critical transition to a
chaotic regime. They suggested that IPCL series might be a manifestation
of the dynamic turbulence of FACs which develops in the cusp region. The difference
in source mechanisms of the noon and morning Pc5 should reveal itself in the
spatial structure (e.g.,
azimuthal phase velocity) of pulsations, which is to be
verified in further studies.

8. Conclusions

The paper is intended to demonstrate that the technique of 1D and 2D mapping
of ground ULF wave activity together with field-aligned current distributions
derived from an ionospheric model and projections of dayside boundary layers
can be an effective tool for a deeper insight into several,
still not resolved, problems of ULF physics.
The preliminary analysis of several events has confirmed the
relationships found in other studies by different techniques,
though these examples have raised interesting questions:

In the morning sector, the region of downward FAC corresponds to the LLBL,
whereas the upward FAC corresponds to the CPS. Thus, the LLBL is a driver of the
R1 current system, at least in the morning sector.

Often, three regions of ULF excitation are simultaneously observed: during
morning hours, near noon, and during afternoon hours. This may indicate the
simultaneous occurrence of several drivers of ULF waves. In the morning sector,
the peak of ULF intensity is commonly located equatorward of the downward R1
FACs, probably in the region of the auroral electrojet, or R2 FAC.

A resonant monochromatic response to Pc5 excitation is observed in the CPS or
near the CPS/BPS interface.
Broadband dayside high-latitude ULF pulsations in the Pc5 range
are commonly referred to as "cusp-related pulsations".
However, the peak of their spatial intensity distribution
coincides with the equatorward boundary of the LLBL.

Regions are identified as one of the following, generally moving from higher to
lower latitudes:
prn, intense polar rain, the suprathermal component of solar wind
electrons;
mantle, de-energized magnetosheath ions observable poleward of
the dayside oval;
cusp, the projection of the magnetospheric exterior cusp, a
region with full intensity magnetosheath ions and electrons;
opll, clearly open LLBL, with low-energy ion cutoffs and
magnetosheath electrons at reduced fluxes;
llbl, closed LLBL with no low energy ion cutoffs, and spectra
closely resembling high altitude LLBL;
bps, boundary plasma sheet where precipitation closely
resembles the poleward portion of the nightside auroral oval. The electrons have
a typical energy of about 300 eV, somewhat higher than in the LLBL;
ps, plasma sheet is the zone of hard electron precipitation with
typical energies above 1 keV on the dayside;
uncl, unclassified: when the flux levels are clearly above
detector noise level but the precipitation did not fit any of the
quantitative rules for other regions;
void, fluxes are generally near or below noise levels.

Acknowledgments

We are grateful to the CANOPUS and IMAGE teams
who kindly provided us with magnetic field data from Canada and
Scandinavia. The help of J. Skura in obtaining the DMSP data is
appreciated. The research of V.A.M.-B. was supported by grant
01-05-64710 from Russian Fund for Basic Research, V.A.P. was
supported by the NATO grant PST.CLG. 978252, M.J.E. was supported
by US NSF grant ATM-9610072, and V.O.P. acknowledges the support
of NSF award OPP-9614175. Helpful comments and suggestions by both
referees are appreciated.